1. Field of the Invention
A solid or semi-solid state electrolyte for alkali metal or alkaline earth metal batteries, and other electrochemical devices, which has been solidified by an alkali metal or alkaline earth metal triflate salt and toughened by partial evaporation of an ether component.
2. Description of the Prior Art
In the prior art various polymers have been used as a component of the electrolytes of solid state alkali and alkaline earth metal batteries, and various other kinds of electrochemical devices.
Among the problems associated with these polymers is that they have inherent relatively low ionic conductivity, and also may react with the component materials, which may be an alkali metal, such as lithium, or other metallic anodes to form a non-conductive boundary layer, or which have a crystaline structure that prevents the free flow of ions, and hinders rechargability.
The prior art polymer containing electrolytes may also exhibit poor adherence to the electrodes, and do not possess sufficient strength to prevent the punching through of dendrites, and consequent shorting of the battery.
The U.S. Pat. No. 5,001,023 to Cheshire et al., describes solid electrolyte devices which include a conductive anode and cathode separated by a solid electrolyte. The solid electrolyte comprises a matrix of polymer main chains, which have side chains linked to the main chains, the side chains comprising polar groups free from active hydrogen atoms. The electrolyte also includes a polar aprotic liquid dispersed in the matrix and an ionised ammonium, alkali metal or alkaline earth metal salt dissolved in the matrix. While the Cheshire patent calls for the addition of lithium triflate to the mixture, it is not used for solidification which he accomplishes by cross linking.
Also Cheshire's method is very slow in requiring a matter of hours for mass production of batteries and the mentioned terpolymer (D1) has side chains and therefore it is not a polyethylene oxide.
In other described examples, Cheshire uses various crosslinked polymers with side chains. Cheshire does not mention the usefulness of his electrolyte for secondary (cylcable) batteries and other electrochemical devices.
The U.S. Pat. No. 5,006,432 to Hope, et al., describes a solid state polymer electrolyte for batteries, where the electrolyte is formed by mixing an ultraviolet light or electron beam curable polymer with an electrolyte, and curing the mixture, whereby the polymer cross links and forms the electrolyte.
The U.S. Pat. No. 5,017,444 to Nakajima, discloses a lithium cell which includes a negative pole of lithium and a positive pole of a vanadium fluoride graphite intercalation compound of the formula CxVF6 where x is from 8 to 80.
While the prior art disclosures may incorporate lithium triflate, they cross link the polymer for solidification of the electrolyte, which does not occur in the present invention. In addition, the prior art devices may not contain all the required compounds, such as polyethylene oxide, as called for in the present invention.
In another example, a liquid electrolyte has been compounded, which consisted of propylene carbonate (PC) and 1,2 dimethoxyethane (DME) and lithium perchlorate salt. However, this liquid electrolyte mixture will not solidify when polyethylene oxide (PEO) is added.
In addition, this and like mixtures will not solidify, if for example alkali metal salts such as lithium hexafluoroarsenate, lithium hexafluorophospate and lithium tetrafluoroborate are used.
While a solid state electrolyte has been made using polyethlene oxide and lithium triflate alone, it did not possess sufficient conductivity, or cyclability, or strength even when propylene carbonate was added.
The solid state polymer electrolytes herein do not suffer from the prior art problems and provide many positive advantages.